The invention relates to the field of optically monitoring deformation, and in particular, checks on structures such as marine flexible pipes or constructions.
Marine flexible pipes are usually used for transporting hydrocarbons extracted from offshore sources. Such pipes are notably described in the standard document API RP 17B “Recommended Practice for Flexible Pipe” published by the American Petroleum Institute. Flexible risers connect a sea bed installation, for example a well head or a header, resting on the sea bed, to a surface installation, for example a floating production unit. A flexible riser has to be able to endure great mechanical stress, notably the stress generated by its own weight, by the internal and external pressures, and by the variations in curvature or bend radius caused by the movements of the surface installation under the effect of the swell and marine currents.
One known solution for reducing the dynamic stresses endured by such risers is to use bend stiffeners that form sleeves and can be fitted around them in order to stiffen them and limit their curvature in the critical zones, notably in their upper part near the surface installation. According to one particular application of these bend stiffeners to risers, they have an upper end secured to the surface installation by means of a flange and extend along the length of the riser over a variable length. The flexible pipe is fitted coaxially into the bend stiffener. Thus, despite the effects of the swell and of the marine currents near the surface, the flexible pipe maintains a radius of curvature which is markedly higher than its minimum acceptable bend radius MBR (which stands for minimum bend (or bending) radius), and thus does not become damaged. These bend stiffeners can also be fitted over portions of pipe near the sea bed in order, once again, to limit their curvature.
Because these bend stiffeners are highly stressed essential components, it is necessary to ensure that they remain intact for a service life which may exceed 20 years. Thus has been conceived the idea of systematically recording the deformations of these bend stiffeners using monitoring devices inserted into their thickness in order to check that they are operating correctly. These measurements can also serve to determine the cumulative dynamic stresses to which the pipe or the bend stiffener has been subjected over the course of time, with a view to estimating its fatigue condition.
Document WO 2005-088375 discloses a device for monitoring the bending of a structure. This monitoring device comprises a deformable rigid rod having a central axis of revolution. The central rod is typically made of glass fiber reinforced epoxy resin. The deformable rigid rod is equipped with three optical deformation sensors which are distributed about and fixed to its periphery. The optical sensors comprise optical fibers which are parallel to the rod and equipped with photo-etched Bragg gratings behaving like optical deformation gages. The three Bragg gratings have identical lengths of the order of a few millimeters. All three are situated on the same axial side along the rod, that is to say that their three centers are included within one and the same plane perpendicular to the axis of the rod. The optical fibers are housed in grooves formed at the periphery of the rod. An optoelectronic device connected to these three sensors can be used to measure the wavelengths reflected by the Bragg gratings and from this deduce the three axial deformations εa, εb, εc experienced by the three optical fibers at said three Bragg gratings. Once these three axial deformations have been measured in this way, it is possible from these measurements by calculation to deduce the three unknowns which are the radius of curvature ρ of the deformable rigid rod in the region of the three Bragg gratings, the angular orientation ψ of the plane of bending with respect to the deformable rigid rod, and finally, the uniform axial elongation ε experienced by the deformable rigid rod, said uniform axial elongation being the result for example of external tensile/compressive stresses or thermal expansion phenomena. This calculation, which is detailed on page 29 of the abovementioned document, also involves parameters assumed to be known regarding the geometry of the deformable rigid rod and of the sensors, and notably relating to the relative position of the three Bragg gratings with respect to the axis of the deformable rigid rod.
The deformable rigid rod equipped with the sensors of such a monitoring device may be embedded within the thickness of a bend stiffener in a part liable to flex, and parallel to the axis of the bend stiffener and to the pipe. Thus, when the flexible pipe is in service, the movements of the bend stiffener cause the deformable rigid rod to flex and thus the sensors supply signals representative of the curvature of the stiffener. On the basis of these signals, the orientation of the plane of bending of the rod and the radius of curvature are calculated. These data can then be processed in real time, for example to trigger an alarm if the radius of curvature or bend radius drops below a predefined critical threshold value or may be logged for later processing, for example in order to estimate fatigue damage and remaining life expectancy.
Such devices are notably described in documents FR2871511, WO2006-021751 and in the publication “Fatigue Monitoring of Flexible Risers Using Novel Shape-Sensing Technology”, reference OTC19051, Offshore Technology Conference, Houston, Apr. 30 to May 3, 2007, which documents also disclose the use of deformable rigid rods of geometries other than cylindrical, for example of octagonal or triangular geometries.
In the publication OTC19051, the deformable rigid rod comprises four optical deformation sensors of the Bragg grating type, the fourth having been added in order to create redundancy. That publication also discloses, on page 2, a method for the calculation of the curvature of the orientation of the plane of bending from the four axial deformations ε1, ε2, ε3, ε4 measured. This method of calculation also involves the relative position of the four Bragg gratings with respect to the axis of the deformable rigid rod.
The bend radii applied to the bend stiffeners vary in practice between a few meters and infinity. In practice, it is necessary for the deformable rigid rods that measure curvature and with which these bend stiffeners are fitted to be able to measure, with accuracy, bend radii ρ greater than 3 m. Because the curvature Cu is the inverse of the radius of curvature or bend radius (Cu=1/ρ), either one of these two magnitudes could be considered, although it is simpler to use the curvature when dealing with the problems of measurement accuracy. The measurement range for curvature is therefore from 0 to 0.33 m−1. The required accuracy for curvature measurement for this application has typically to be better than +/−0.0015 m−1, this level of accuracy being needed in order to perform fatigue damage analysis.
In addition, to facilitate the manufacture, storage, transport and installation of these deformable rigid rods, which may have an overall length in excess of several tens of meters, it is desirable for these to be able to be wound up with a minimum radius of curvature of the order of 0.5 to 1 m. Now, the optical fibers with which the deformable rigid rods are equipped must not be subjected to relative elongations in excess of 0.5% because if they are, there is a risk that they will become damaged. This is why the deformable rigid rods intended to be fitted to the bend stiffeners of offshore flexible pipes in practice have a small diameter, typically of the order of 5 mm to 10 mm, which makes it possible to reduce the maximum elongations experienced by the optical fibers when the deformable rigid rod is bent to its minimum bend radius (see page 25 of WO 2005-088375). This small diameter does, however, have the disadvantage of contributing toward reducing the accuracy of the curvature measurements, so that the objective of accuracy to within +/−0.0015 m−1 is, in practice, particularly difficult to achieve when the diameter of the deformable rod is of the order of 10 mm or less.
Another aggravating factor in terms of this measurement accuracy problem is that the deformable rigid rod is made of glass fiber reinforced resin, which means that not-insignificant spread is introduced into various geometric parameters: the diameter of the rod, the shape of the grooves, the angular offset between the optical fibers, the distance between the optical fibers and the central axis of the rod, etc. Using epoxy resins to fix the optical fibers to the periphery of the rod also generates stresses which can become the root cause of measurement errors. Such composite materials, because of their method of manufacture, generally have mediocre dimensional tolerances, of the order of plus or minus a few tenths of a mm. These mediocre tolerances also give rise to errors which are all the greater, the smaller the diameter of the rod. Thus, in practice, in the case of a deformable glass fiber rigid rod of 10 mm diameter, it has been found that the error on the curvature measurement can often exceed +/−0.003 m−1, and this is not satisfactory.
One first solution for solving this problem is systematically to test the measurement accuracy of each deformable rigid rod after it has been manufactured, and to keep only those which achieve the required level of accuracy. However, this solution gives rise to a high scrappage rate and is not economically viable.
A second solution is to produce the deformable rigid rod from a metal section piece with a high elastic limit, that has been shaped with a high level of accuracy, for example a titanium section piece obtained by cold drawing and which has dimensional tolerances to within one hundredth of a millimeter. However, this solution is not very economically attractive.
A third solution for solving this problem is to increase the diameter of the deformable rigid rod, as taught in publication OTC19051, that document proposing a diameter of 75 mm. This solution improves the measurement accuracy for large bend radii. However, it has the disadvantage of increasing the minimum bend radius that the deformable rigid rod can tolerate without the risk of damage to the optical fibers, and this on the one hand poses problems regarding the storage and handling of said rod, and on the other hand prevents small bend radii from being measured. In practice, a deformable rigid rod with a diameter of 75 mm ought to be stored on 15 m diameter reels and could not be used, without the risk of premature fatigue failure, for durably measuring bend radii smaller than 15 m to 20 m, these being radii that the bend stiffener is likely to reach only in very heavy weather or storm conditions.